Olefin polymers are particularly subject to severe deterioration from the oxidative action of air at elevated temperatures. They are also vulnerable to degradation by the action of ionizing radiation when large amounts of such radiation are deposited into the polymer, e.g., amounts of 5 .times. 10.sup.10 ergs per gram and higher. Oxidative deterioration at elevated temperatures is manifested by surface crazing, then cracking and ultimately a complete embrittlement.
It is known that small amounts of some antioxidants, e.g., 4,4'-thiobis(6-5-butyl-m-cresol), can be added to polyolefins and more particularly to polyethylenes to prevent or impede these degradative effects without the contribution of color to the polymer. However, such conventional antioxidants are not effective under conditions of high vacuum and/or high temperatures since with time the antioxidant volatilizes away from the substance into which it was incorporated.
Until recently, the volatility of such antioxidants in a crosslinked polyolefin composition was undesirable primarily because of the reduction in oxidative stability which occurred as the antioxidant volatilized away.
The use of crosslinked polyolefin materials in space vehicles has brought to light a still more important disadvantage of these antioxidant materials. When used in a space vehicle, a crosslinked polyolefin material, employed for example in the form of electrical wire installation on solar panel wiring or in front of tubing, may be exposed to solar radiation in the hard vacuum of space such that the polyolefin may be heated to temperatures as high as 125.degree. to 150.degree. C. Under such conditions, most of the prior art antioxidants, such as those previously mentioned, will be volatilized away. Since optical systems in the space vehicle, shielded to solar radiation, may be at a relatively low temperature, perhaps 10.degree. C., the gas formed from the antioxidant will condense on the optical system thereby adversely effecting its transmitive and reflective optical characteristics. Furthermore, the heated polyolefin with its antioxidant-radiation protectant volatilized away is now vulnerable to radiolytic degradation such as that which may be caused by energetic solar protons.
It is also known that certain polymeric involatile materials may be used as antioxidants. Thus, certain phenolformaldehyde polymers may be used as antioxidants. Also, bisphenols and triphenols such as those described in U.S. Pat. No. 3,247,262 may be used to protect against oxidative degradation. The bis and triphenols are still volatile, however. The phenol-formaldehyde resins, which have lower volatility, have disadvantages including thermosetting characteristics of the phenol-formaldehyde antioxidant, choice of precursors on the basis of synthetic convenience rather than antioxidant capability and a pronounced tendency to impart coloration to polymers to which they are added.
U.S. Pat. No. 3,133,899 to Kwiatek proposes the copper chloride-catalyzed oxidative polymerization of a thiobisphenol to form polymeric ethers of formula ##SPC1##
the thiobisphenols employed having the formula ##SPC2##
wherein at least one R on each phenolic group is alkyl, the other being alkyl or hydrogen, alkyl in every case preferably having 1 to 3 carbon atoms. Should those R groups be of more than 3 carbon atoms, the work of Hay et al. in J.A.C.S. 81,6335 (1959) with oxidative coupling of 2,6-disubstituted phenols would suggest that the Kwiatek procedure would yield diphenoquinones as the sole product. Such materials form undesirable color bodies when blended in olefinic polymers and the like.
Braus et al., in U.S. Pat. No. 3,462,375 disclose antioxidant compositions of formula ##SPC3##
but the values assigned Z.sub.1-2, Y.sub.1-3 and M.sub.1-3 make clear that it is a phenol-substituted benzene compound rather than a bisphenolic polymer which is proposed. Again, in U.S. Pat. Nos. 3,377,333 and 3,044,960, respectively, to Ciesielski et al. and Norway et al. alkyl-substituted thiophenolic polymers are disclosed whose repeating units are, e.g., ##SPC4##
Such polymers are distinguished from the bisphenolic polymers of the present invention by, inter alia, the characteristic lack in the former of direct carbon-carbon bonding between the phenolic rings of adjoining thiobisphenolic repeating units.